During the operation of continuous ink jet printers it is well known to stimulate or modulate the jet or jets so that they are perturbed and break up into uniformly sized and evenly spaced droplets. To achieve acurate droplet charging, it is important that the droplet stream is satelite free and that the break up point is both stable and occurs within the charge electrode. In a multi jet system there is a further requirement that each jet has near identical break up characteristics, i.e. the break up length, the break up phase and the break up shape are similar from jet to jet.
The necessary jet modulation is, in one conventional technique, achieved by using an acoustic generator to transmit an acoustic wave into a body of ink in an ink cavity one side of which is closed by a nozzle plate through which the or each jet is discharged. The acoustic generator usually consists of or includes a piezoelectric actuator.
With single jet systems, the acoustic generator is usually positioned at the side of the ink cavity opposite to the nozzle plate and sufficient energy is provided to force vibrations of the ink in the direction parallel to the jet, that is perpendicular to the nozzle plate, to cause the jet to break up into droplets. No attempt is made to control vibrations in a direction perpendicular to the jet, as these have no signicant effect upon the operation of the system. In multi-jet systems, (as disclosed for example in GB-A-1464370) the ink cavity may be divided into separate compartments each associated with one acoustic generator and one nozzle, and this is effectively an array of single jet systems.
In other multi jet systems a single acoustic generator is provided for a substantially linear array of nozzles and in this case it has been recognised that it is necessary to stimulate in the ink cavity substantially only "longitudinal" vibrations, parallel to the jets, substantially without any "transverse" vibrations perpendicular to the jets, as these would produce unwanted transverse variations in pressure amplitude. In order to achieve this, large acoustic generators having a dimension, parallel to the linear array of nozzles, greater than the length of the array of nozzles have been used and complicated techniques, such as cutting a block of piezoelectric material into a comb like shape and attaching it to a membrane (as disclosed in U.S. Pat. No. 4,668,964) have been used in order to minimize the production of transverse waves. In other words it has been assumed that each nozzle in the array must be aligned immediately opposite either its own acoustic generator or its own part of an acoustic generator having a dimension parallel to the length of the array which is greater than the length of the array.
Surprisingly, the inventors have now found that this is not necessary and that, provided certain resonant conditions are satisfied, a single acoustic generator can satisfactorily stimulate the ink passing through an array of nozzles, or a part of an array of nozzles, which has a length considerably greater than the dimension of the acoustic generator parallel to the length of the array or part array.